Optical fibers are well known in the art of light transmission along a length of filament by multiple internal reflection of light. A single-core optical fiber usually consists of a core and a cladding which has an index of refraction lower than that of the core. A multi optical fiber bundle, often referred to as an image guide, can be effectively considered as a bundle of single optical fibers where each fiber transmits a light signal independent of the others. Optical fibers may be fabricated from glass or plastic.
One of the largest applications of a two-dimensional image guide is for transferring images. Because each individual fiber of the optical image guide transmits only a minute discrete portion of the image, it is of course preferred for each point of the end (face) of the image guide to be coherently related to the corresponding point on the other end (face) such that the image exiting the image guide is identical to that which enters the multiplicity of the fibers. Image resolution, brightness and contrast sensitivity are often used to characterize the image quality after transmission through the image guide.
U.S. Pat. No. 4,271,104 (1979) discloses a method for producing optical fiber ribbon. The method incorporates a single material and utilizes hot rolling and extrusion techniques. In order to form a plurality of adjacent fibers in the ribbon, longitudinal separation of fibers is made by reducing (or increasing) the refractive index at parallel lines delineating the array of fibers. Refractive index modification is achieved by applying strain to induce the photo-elastic effect, depositing barrier masks by using photolithography, thermal diffusion, ion exchange, and/or ion implantation. A disadvantage of this method is its complicated and inefficient way of achieving the effect of cladding. As mentioned in U.S. Pat. No. 4,271,104, the cross-talk between the individual fibers cannot be eliminated. In addition, this method cannot be efficiently utilized for making optical arrays or matrices with very fine cores. It is essentially a method for making preforms, which need to be drawn down in a later stage in order to reduce the size of the individual cores. The effective cladding achieved in this way can be seriously deteriorated in the drawing process.
Methods for producing polymer optical fiber matrices is described in a series of patents issued to Mitsubishi Rayon Company of Japan, for example U.S. Pat. No. 4,812,012 (1989), U.S. Pat. No. 4,842,365 (1989), EPO 0427232A2 and U.S. Pat. No. 5,127,079 (1992). These patents disclose co-extrusion of a core polymer material, a cladding polymer material, and sea polymer material through a multi-hole die to make the fiber matrix directly. This is a very efficient method for making small size, for example, less than 3 mm in cross-sectional dimension, multi-core optical fibers or plates. In order to make multi-core optical fibers or plates with bigger cross sections, many such small matrices have to be stacked and fused together in a later stage. A seamless image requires that fiber-to-fiber spacing inside a matrix be equal to fiber-to-fiber spacing across a seam between matrices. This requirement is not met by the Mitsubishi production technique which teaches a significant sea polymer thickness around the periphery of a matrix. The result of this stacking process on the transmitted image is to produce an image which has a pattern of seams corresponding to the interfaces between the matrices.
Furthermore, it is desired to manufacture polymer optical fiber matrices with very high production rates. The Mitsubishi patents cited above pertain to symmetric two-dimensional arrays of fiber matrices. Limited production rate due to difficulty in cooling a symmetric matrix of optical fibers is a significant limitation of the Mitsubishi patents.
Another limitation of the Mitsubishi patents is the use of step-index optical fiber which can reduce the light transmission at small microfiber diameters. Specifically, U.S. Pat. No. 4,842,365 in Table 2 teaches that the transmission loss (dB/m) is 2.3, 1.6, 0.91, 0.70 for diameters of 16, 28, 180, 225 micrometers, respectively. These measurements were made in the red region of the visible spectrum. The loss can be expected to be about a factor of two greater for blue light. The difference in loss for different colors can lead to image color distortion. Since the length of the light path in step-index fiber is independent of the fiber diameter, light absorption by the core material is independent of the fiber diameter. For this reason, the change in light transmission as a function of the fiber diameter is generally conceded to be due to imperfections at the core/cladding boundary.
In particular, the light will interact with the core/cladding boundary a greater number of times over the same length of fiber for smaller core fibers as compared with larger core fibers. This interpretation is buttressed by the observation that a single graded-index plastic optical fiber with diameter of 5.0 μm has been measured to have a transmission loss of 0.2 dB/m (Koike, Y. et al. (1993) in Design Manual and Handbook and Buyers Guide, Information Gatekeepers, Inc., Boston, p. 19). This measured transmission loss is about the same as is measured for fiber diameters of about 1.0 mm, implying that the graded-index fiber loss is dominated by material absorption unlike the case of small diameter step-index fibers. As a result of these considerations, existing image guides using step-index plastic optical fibers compromise both the optical transmission and the fidelity of image color.
In order to achieve better light transmission and improved resolution, U.S. Pat. No. 5,881,195 (1999) disclosed a plastic image guide comprising a plurality of gradient-index (GRIN) optical fibers. Single-core GRIN fibers are stacked, fused, and drawn to make a plastic image guide. One technique taught for attaining the variation in refractive index involves mixing two miscible transparent polymers with different refractive indices in a manner which produces a radial parabolic refractive index profile of the individual fibers. The use of this method is limited by the polymers which can be used. (1) the polymers must have good light transmission and have enough difference in refractive indices; and (2) the polymers must be compatible, for example miscible at the molecular level, in a wide range of mixing ratios. Nevertheless, image guides made in this way appear to have high resolution and good transmission. However, because this production method involves stacking, fusing, and drawing a plurality of single fibers, a practical limitation of about a 3 mm2 transverse area for the image guides and low volume manufacturing capacity can result.
Single graded index plastic optical fiber can be produced by several known techniques, for example as described in U.S. Pat. No. 5,593,621. Many of these methods utilize batch production techniques in which a fixed quantity, typically a kilogram at most, of material is used to produce fiber. Accordingly, these methods can have limited production capacity.
Many researchers are in pursuit of a television set which can hang on a wall like a picture and be, at most, a few inches thick. A promising technology in this field today is the liquid crystal display (LCD). However, the following discussion also applies to plasma displays (PDs), field emission displays (FEDs), electroluminescent displays (ELDs), organic light-emitting displays (OLEDs) and digital mirror displays (DMDs). Liquid crystal displays have been commercially available for more than 20 years, but until recently have been restricted to a relatively small size. Recently, improvements in liquid crystal technology have occurred, allowing larger, high-line density displays to be manufactured.
One recent technique involves the addition of active switches to control the action of the liquid crystal at each picture element or pixel. The active switch can be a thin film diode or a thin film transistor. These displays are typically termed “Active Matrix Liquid Crystal Displays,” or “AMLCD.” These AMLCD's can achieve higher speed, higher contrast, and/or higher overall brightness. The use of these active devices to control “light valves” can greatly simplify the electronics of the display, but can also require one or more added fabrication sequences to deposit the active devices at each pixel. In addition, there is a need for leads to each device, such that on the order of hundreds of thousands of devices for each display may require leads.
The typical keys to the economical production of AMLCD's are the yield of the complex sequential process and the number of displays which may be cut from a panel. Currently, second generation panels are currently available up to around 21 inches, while third generation panels, expected some five years in the future, may be as much as 29 inches. Sizes much larger than 30 inches are not expected before the year 2010.
To reach larger sizes, where monolithics are not economical to produce due to the problems discussed above, a plurality of relatively small “tile” AMLCD's can be connected together in precise alignment to form a large display. Such displays are typically characterized by visually disturbing seams resulting from gaps between adjacent pixels on adjacent tiles. Thus, the image portrayed by using a seamed display often appears segmented and disjointed. Therefore, it is desirable to fabricate a tiled, flat-panel display which does not have noticeable, or even perceptible, seams.
The pixel pitch in electronic displays is set so that the minimum viewing distance will produce an imperceptible seam between pixels. With a standard pixel pitch P=0.26 mm, the minimum viewing distance is on the order of one meter. The minimum viewing distance will increase with the pixel pitch; therefore, when designing for the purpose of visually eliminating the seams, there is very little latitude in the selection of pixel pitch. For a seamless multi-tile display, it is generally agreed that at the intersection of the tiles, the edge dimensions thereof are preferably maintained so that the interpixel spacing remains uniformly periodic throughout the tiles and across the seams.
A frame can surround the edge of the glass panels containing the AMLCD's. Seals for liquid-crystal displays are generally located at the perimeter thereof and can be covered by the frame. Such seals can provide a mechanical joint between the top and bottom glass plates of the flat-panel displays (FPD), as well as containing the liquid-crystal material between the plates. The widths of the seals themselves are customarily a few millimeters. When space is allowed for electrical connections, a typical non-luminous width around a standard production AMLCD may be about 6 mm, or about 7 mm including tolerance on dimensions.
The seals are usually polymeric adhesives and are usually epoxy-based, thus having a solubility for water and a diffusivity that is appreciable. The rate of diffusion increases exponentially with the reciprocal of the width of the seal, as well as in proportion to the seal thickness and the diffusivity constant for the seal material. Seal width is a major contributor to seam width, since there are two seals in a seam width, i.e., one on each tile perimeter. The desired width of seals for individual AMLCD tiles for direct tiling purposes is less than about 0.1 millimeter to eliminate perceptible seams; however, the seals for AMLCD FPDs have only been proven to be reliable for widths of one or two millimeters or wider. This contradiction in requirements is a major problem associated with the existing technology of AMLCD tiles.
There have been at least two general approaches to producing a tiled display with imperceptible seams. The first of these approaches requires the development and production of special tiles with extremely narrow seal widths which must be no more than about 0.15 mm. A number of patents (U.S. Pat. No. 5,889,568, U.S. Pat. No. 5,668,569, U.S. Pat. No. 5,867,236 U.S. Pat. No. 5,963,281, U.S. Pat. No. 5,781,258, U.S. Pat. No. 5,903,328, U.S. Pat. No. 5,593,621) have been filed to describe methods of making invisible seams between tiles according to the above methods and requirements. Despite considerable effort, there does not appear to be any successful efforts in making two-dimensional arrays of seamless tiles according to these methods.
An alternative approach to producing seamless tiled arrays has been described in several patents, U.S. Pat. No. 4,299,447, U.S. Pat. No. 3,909,109, U.S. Pat. No. 4,139,261, U.S. Pat. No. 4,786,139, U.S. Pat. No. 3,853,658, U.S. Pat. No. 5,465,315, and U.S. Pat. No. 5,129,028. These patents describe the use of standard tiles and different architectural designs of fiberoptic structures to eliminate the appearance of seams between the tiles. It does not appear to be practical, or commercially feasible, to fabricate the fiberoptic structures described in these patents. In addition, these fiberoptic structures produce images having contrast which may be inadequate for general use.
Thus there is a need in the art for a means of optically masking the unwanted seam grid created between adjacent display modules arranged in a mosaic array which is practical, economical, and has good image quality.